The vibration energy harvesting beam described in the '117 application attempts to maximize the strain of bonded piezoelectric patches and maximize the electrical output by providing a slotted, tapered vibrating beam that places the piezoelectric patches away from the neutral axis of the beam. Such a vibrating beam is especially useful when the ambient vibration level is low and if the vibrating beam may be tuned to be resonant at the predominant frequency present in the instrumented component, machine, or structure to which it is mounted. Such an energy harvester was tuned to generate electricity to power a wireless temperature and humidity sensing node from ambient vibration, as described in the '840 application.
However, in many cases the ambient vibration level may be much higher but the predominant frequency may be inconsistent or unpredictable. For example, aboard helicopters the predominant vibration frequency may be the rotational rate of the rotor assembly times the number of rotor blades in the assembly. Thus, the structure of the Sikorsky H-60 helicopter, which has four rotor blades and has a typical rotational rates of about 4.3 Hz has a predominant vibration frequency of about 16-17 Hz. The G levels have been reported to vary significantly with location from about 1 to about 5 G's. Other rotating structures on this helicopter experience fundamental vibration frequencies that may be lower, such as the pitch links or control rods, which vibrate with the rotational rate of the rotor assembly of about 4.3 Hz, but which also contain higher frequency components. What is needed is an energy harvester design that will generate electricity efficiently under a wide range of vibration amplitudes and frequencies.